1. Field of the Invention
This invention relates to a method of producing an optically non-linear ionic crystal for high output laser. More particularly, the invention relates to an electrodialytic method of growing water-soluble ionic crystal such as potassium dihydrogen phosphate (KDP), deuterated potassium dihydrogen phosphate (DKDP), and ammonium dihydrogen phosphate (ADP).
2. Related Art Statement
Various methods have been proposed for the growth of water-soluble single crystal; namely, a temperature falling method, an evaporation method, a three-vessel constant-concentration method, an electrodialytic method, and so on. The inventors previously disclosed an electrodialytic method which had numerous advantages; for instance, the ease of control of the crystal growth rate through regulation of an electric current alone while maintaining a growth tank at a constant temperature, the availability of crystal material supply without interruption of the crystal growth, and the smallness of the growth tank.
A conventional electrodialytic method of growing ionic crystal will be described by referring to FIG. 2 which illustrates production of KDP crystal. A growth tank 51 of the figure is divided into five vessels A through E which are separated by anion-exchange membranes .alpha..sub.1, .alpha..sub.2 and cation-exchange membranes .beta..sub.1, .beta..sub.2. The five-vessel structure of the growth tank 51 is used because the pH of solutions in end vessels A and E varies with the growth of the desired crystal in a crystal growth vessel C and the provision of side vessels B and D therebetween prevents such pH variation from exerting adverse effects on the function of the crystal growth vessel C. A positive electrode 52 is disposed in one end vessel A, while a negative electrode 53 is disposed in the other end vessel E so as to apply an electric current to the growth tank 51.
After a solution of KDP is filled in all the vessels A through E of the growth tank 51, if an electric current is applied across the electrodes 52 and 53, cations (K.sup.+) move from the side vessel B to the crystal growth vessel C while anions (H.sub.2 PO.sub.4.sup.-) move from the side vessel D to the crystal growth vessel C. Thus, KDP is concentrated in the crystal growth vessel C while it is diluted in the side vessels B and D. At the same time, cations (K.sup.+) move from the side vessel D to the end vessel E while anions (H.sub.2 PO.sub.4.sup.-) move from the side vessel B to the end vessel A. In the end vessel A, supersaturated anions (H.sub.2 PO.sub.4.sup.-) are consumed at the positive electrode 52 to produce oxygen O.sub.2, while in the other end vessel E, supersaturated cations (K.sup.+) are consumed at the negative electrode 53 to produce hydrogen H.sub.2. If one or more seed crystals 54 of KDP are placed in the crystal growth vessel C, the supersaturated KDP therein crystallizes on the seed crystal 54. Thus, the desired growth of KDP crystal is achieved. Supply of additional KDP powder at the side vessels B and D in compensation for the crystallization will enable continuous growth of such KDP crystal.
Such conventional electrodialytic method has a shortcoming in that, since all the vessels A through H are kept at the same temperature in a range of 20.degree.-60.degree. C. or at a room temperature, spurious crystals are deposited on the ion-exchange membranes .beta..sub.1 and .alpha..sub.2 at portions where concentrations of ingredients become high during the KDP crystallization. The spurious crystals deposited on the ion-exchange membranes also grow, and in the worst case, they cover the entire membrane surfaces and actual growth of the desired crystal in the crystal growth vessel over a long period of time becomes impossible.